The investigation of the motion of electrons and atoms in nanostructures requires ultrafast measurement techniques with a high sensitivity to tiny sample volumes. Low-energy electrons have the highest scattering cross section and interact strongly with electric and magnetic fields. During her PhD studies, Melanie Müller developed a novel ultrafast electron microscopy technique based on femtosecond single-electron wave packets emitted from a sharp metallic needle. Utilizing this technique, Melanie demonstrated that the photocurrent arising inside an InP nanowire after optical excitation e can be filmed with femtosecond resolution. Melanie received her PhD with distinction.

new paper: Waldecker et al., Physical Review B 95, 54302 (2017).
The semimetal antimony is a model system for studying electron-lattice correlation and coherent phonons. The electronic structure of antimony induces a static lattice distortion, a so-called Peierls distortion. Optical excitation of the electrons with a short laser pulse impulsively reduces the mechanism of the Peierls distortion, leading to a collective oscillation of all atoms in the crystal. In addition, the energy given to the electrons by the laser pulse dissipates by incoherent scattering of the electrons with all other lattice vibrations. Applying our femtosecond electron diffraction apparatus, we were able to simultaneously observe and distinguish both phenomena.

new paper: Bertoni et al., Physical Review Letters 117, 277201 (2016).
A range of transition metal dichalcogenides (TMDCs) are semiconductors with layered crystalline structure. In the form of monolayers, these TMDCs are 2D materials with peculiar optoelectronic properties and an unusual spin texture of the electronic structure, i.e. a spin-valley correlation. In contrast, bilayers and bulk crystals of 2H-WSe₂ are centrosymmetric, which causes all electronic states to be spin-degenerate. We show, however, that spin-polarized excited carriers can be generated in bulk crystals of the non-magnetic layered material WSe₂. This is a consequence of the hidden spin polarization in this class of materials: optical excitations generates excited states with 2D character, i.e. localized to an [more...]

Lutz Waldecker receives the Carl Ramsauer Award of the Physikalische Gesellschaft zu Berlin (PGzB) in recognition of his PhD thesis Electron-Lattice Interactions and Ultrafast Structural Dynamics of Solids. The price is awarded for outstanding doctoral research studies in physics or related fields at the three Berlin universities and the University of Potsdam. Congrats, Lutz!

In collaboration with researchers at the MPI for Quantum Optics, both Munich universities and Monash University, Australia, the generation and control of electric current in a semiconductor on time scales short than the oscillation period of visible light (~2 femtoseconds) has been demonstrated. This study extends the previously achieved current control in dielectrics to the material class of semiconductors. The study reveals a crossover in the mechanism of current generation from the multiphoton to the tunneling regime depending on the intensity of the employed laser pulses.
Publication: Paasch-Colberg et al., Optica 3, 1358 (2016).

The European Research Council (ERC) funds the 5-year project FLATLAND with 2.6 million €.
FLATLAND is an experimental research project addressing the exotic spin-valley-layer correlations in few-layer thick transition metal dichalcogenides (TMDC) crystals and related heterostructures. Microscopic coupling and correlation effects, both within pure materials as well as across the interface of heterostructures, will be accessed by time- and angle-resolved extreme ultraviolet-photoelectron spectroscopy, femtosecond electron diffraction, and time-resolved optical spectroscopies. The project promises unprecedented insight into the microscopic coupling mechanisms governing the performance of van der Waals-bonded devices.

At room temperature, titanium diselenide is a metallic crystal with pronounced electron-electron as well as electron-phonon correlations. At low temperature, a metal-insulator phase transition occurs concurrently with the formation of a charge density wave and a periodic lattice distortion. In collaboration with the research group Dynamics of Correlated Materials, we investigated the response of the electronic structure of this material to infrared and infrared optical excitation. The transient electronic structure was measured with time-, energy- and momentum-resolution by time- and angle-resolved photoelectron spectroscopy (trARPES) employing femtosecond extreme-ultraviolet laser pulses.
full publication: Monney et al., Phys. [more...]

Michele Puppin receives the Best Poster Prize at IMPACT 2016 (Electronic States and Phases Induced by Electric or Optical Impacts). The poster describes how to map the conduction band of a semiconductor in the whole Brillouin zone with the help of high-repetition rate XUV time- and angle-resolved photoemission spectroscopy (trARPES). The study directly visualizes the valence and conduction bands of the bi-dimensional semiconductor WSe₂, a member of the transition metal dichalcogenide family.

Lutz Waldecker developed a femtosecond electron diffractometer and employed this apparatus to study Electron-Lattice Interactions and Ultrafast Structural Dynamics of Solids. He received his PhD with distinction from FU Berlin.

new paper: Waldecker et al., Phys. Rev. X 6, 021003 (2016)
We revisited a basic problem: the exchange of energy between electrons and phonons in a simple metal like aluminum. Typically, this energy flow is described by the two-temperature model. We show that this model needs to be refined and propose the non-thermal lattice model.

new paper: Müller et al., ACS Photonics 3, 611 (2016)
We report a point source of single electron wave packets driven by plasmons. The duration of the electron wave packets is less than 10 femtoseconds. This source enables femtosecond electron holography, and potentially even femtosecond scanning tunneling microscopy.
This work was performed in collaboration with the group of Markus Raschke at the University of Colorado.

We developed a high-repetition rate extreme ultraviolet (XUV) laser delivering 500,000 pulses of photons with 22 eV energy and a duration of approximately 20 femtoseconds. These light pulses are used to photoemit electrons from a crystal in order to obtain a map of the material’s electronic structure. As we use a second, visible laser pulse to excite a small fraction of the material’s electrons into excited states shortly before the XUV pulses arrive, we are now able to do excited state mapping and to take movies of how electrons scatter in the band structure.
This achievement required years of development of new laser sources (see Puppin et al., Opt. Exp. 23 1491, 2015), high-harmonic generation, and a sophisticated surface science apparatus. This novel technique is developed in close collaboration with the [more...]

new paper: Waldecker et al., Nature Materials 14, 991 (2015)
No need to read. There is an audiovisual summary of this work.